FirstLight Astronomy Club

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Putting Light to the Equinox

Of all the big four seasonal dates - the winter and summer solstices and the spring and fall equinoxes - the fall equinox falls under “Most Ignored.”

Winter solstice, the shortest day of the year, is famed throughout the world in nearly every culture and is celebrated in a wide variety of festivals. Summer solstice is the longest day of the year, marking our first day of summer, a high holiday for most American school kids. The spring equinox denotes for many the end of winter, and the beginning of both new life, and the allergy season.

But most of us pass right by the fall - or autumnal - equinox with nary a notice. So our column this week is dedicated to this unsung seasonal date that we will celebrate on the 22nd. It’s time the equinox got equal time.

So what is this fall equinox all about anyway?

Our planet, as you learned in school many years ago, travels around the sun in a nearly circular orbit. And we all know that the Earth spins with a slight tilt to it.

Well, we spin about smoothly enough on our axis each day, almost like the perfect top, and revolve slowly and effortlessly around the sun year after year. But what confuses some people is that our planet always has its spin axis pointing in the same direction – all year long. For us in the northern hemisphere, our pivot point points towards Polaris, aka The North Star.

At winter solstice we in the northern hemisphere are tilted away from the sun. Therefore, we have shorter days and cooler temps, and the sun hangs low in the sky.

The summer solstice is when we are tilted most toward the sun and it starts to get noticeably hot and the daytime is longest.

But right between those two days, once in March and once six months later in September, we are tilted neither towards nor away from Sol.

The September date is this Thursday. Since we are not tilted with respect to the sun, our “daytime” will have 12 hours with the sun being above the horizon, and 12 hours when it will be below. Hence the term equinox, from the Latin æquinoctium, from “æqui” meaning equal, and “nox” meaning night, inferring equal times of day and night.

Because the earth is standing at virtual attention to the sun, there will be light from pole to shining pole. That means essentially everyone on this planet will get 12 hours of sun up, 12 hours of sundown. It is an equal opportunity day.

But the equinox is more than just 12 up, 12 down.

The sun on this day rises due east and sets due west for everyone. And I mean the real east and the real west! None of that funky compass stuff.

The compass faces toward a magnetic North Pole, which is not true north for just about everyone on Earth. Thus, neither is the “E” of the compass true east, nor its “W” true west.

But on the day of the equinox, if you have a nice flat horizon you can mark true east and true west truer than any compass by observing exactly where the sun rises and sets.

Now I sort of didn’t tell the whole truth when I told you that all people on earth would get 12 hours of sun up, and 12 hours of no sun. There are two places that are exempted here. Can you guess what those places might be?

I’ll give you a hint. At the equator, one will see the sun rise due east, travel straight up over head then straight down, and set due west.

Here in southern California we will see it rise due east at an angle, sweep over the southern skies at noon (only about 57 degrees above horizon at its height), and set at a sharp angle due west.

Higher north in Canada the sun rises due east, and sharply cuts through the low southern skies, setting 12 hours later due west.
Any ideas yet of where the sun neither rises nor sets on this day?

At the poles. At the North and South poles, the sun will not rise or set! One standing there will actually see the sun just skirt the horizon in a giant circle, never rising nor setting.

For us who live here in southern California it would be a quite a strange sight indeed!

Here are some things you can do Thursday.

In the morning you can mentally mark the place on the horizon where the sun rises (assuming no hills!) and truly call that East. Twelve hours later you can see where it sets, again assuming a flat horizon, and mark that as West.

Then watch in the next weeks as we head toward the next seasonal marker, the winter solstice, how the sun rises more southerly, moves lower through the sky, and sets more southerly. All this means less sunlight, lower temperatures, a lower sun, and more annoying sun-in-the-eyes on your drive to work.

Have an awesome equinox!


Tonight there’s likely to be a bluish-white Vega parked right outside your house. Go ahead and step out and look to the left or to the right; you won’t see it. You’ll have to look up – straight up.

Almost directly above your heads during this time of the year is a bright star in the constellation Lyra. It is Vega, derived from the Arabic name of the constellation, Al Nasr al Waki, which means Swooping Eagle. Philologically speaking, Al Nasr al Waki begat Wega. Wega begat Vega. And there you are.

Let’s look under the hood of this star and see what makes it run.

Vega is 25.3 light years away, about 150 trillion miles as the crow flies. But that’s actually pretty close considering our galaxy alone is well over 100,000 light years across! Why, Vega’s almost a neighbor.

Vega is just over three times the size of our sun. That works out to a good 2.8 million miles through, which is one hefty bag of plasma. It’s all this extra mass that makes it burn a whole lot more efficiently than our own sun.

As a result, Vega gushes more than 60 times the energy that our star does, and a lot of that energy is from the destructive ultraviolet side of the electromagnetic spectrum.

Stars similar in brightness to Vega join her in making up the distinctive Summer Triangle. Bright Altair and Deneb both round out this pointy asterism that can be seen high in the heavens for a couple more months.

Vega also has the distinction of being the first star photographed by Earth-based paparazzi. In 1850, William Cranch Bond and John Adams Whipple, in a day when it was apparently stylish to use middle names, were the first to image the starry object.

On the night of July 16/17 they photographed Vega with the telescope at Harvard Observatory using the early daguerrotype process. But Vega was not content with that brief bit of notoriety. Over 100 years later, she would play yet another leading role in astronomical imaging.

Vega rotates, as all stars do. But we see it pole-on, looking down on it as it spins. New, improved images of Vega reveal to us that the star is surrounded by… well, a whole lotta crud. Since 1983 we’ve been looking down at the full face of a big dirty spinning Frisbee, with bright Vega right in the middle of it, as if she were brooding over a solar system in the making.

And just recently it was discovered that some of this crud is being blown away by the solar winds of the mother star. This present sweeping implies there was a recent rocky collision there. Could it be that two young planets, possibly the size of Pluto, have recently collided and vaporized into massive clouds of dust?

And the fate of Vega? Because it is such a big star it will exhaust its fuel much faster than our star. It’s really only a relatively young stellar object – 350 million years old, plus or minus – considerably younger than our 5 billion-year-old sun.

But according to present theories of star formation, Vega will retire from stardom in about 650 million years, fizzling out into a nondescript white dwarf. It will be lucky if it sees its one billionth birthday.

But before she fades away Vega will go out with at least one more crowd pleaser.

Because the Earth wobbles slowly on its axis, the North Star is not always the one we see now, which is Polaris. In about 10,000 more years we will have wobbled around enough such that the new North Star will be – you guessed it! - our star of the month, Vega.
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me